Mechanical peristaltic pumps are well known. By way of example, such pumps may be used for the infusion of medical fluids into, or for the removal of body fluids from, a patient. These systems typically comprise a length of flexible tubing within a compression chamber defined by a compression surface and a rotor assembly. The rotor assembly includes a plurality of parallel rollers disposed about the periphery of a rotor. With the rotor assembly rotating, the rollers are biased against the flexible tubing that is backed up by the compression surface. The rollers successively pinch off the tubing, advancing the pinch-off position so as to progressively move the fluid within the tube and into or from the patient's body at a controlled rate that is determined by various design and operational parameters such as the angular velocity of the rotor assembly.
Electroactive polymer (EAPs), also referred to as electrically conductive or conducting polymers, are flexible materials capable of converting energy in the form of electric charge and voltage to mechanical force and movement. Thus, these materials are able to change shape in response to electrical stimulation. Common electroactive polymers include polyaniline, polypyrrole and polyacetylene. It is well known that dimensional changes may be effected in these polymers by the mass transfer of ions into or out of the polymer that causes expansion or contraction of the polymer.
Ionomeric polymer-metal composites (IPMC) comprise a subcategory of ionic EAPs. The detailed description, below, will be directed toward these but the invention is not limited to them.
A typical IPMC consists of a thin (200 micrometers) polymer membrane with a metal electrode (5-10 micrometers thick) plated on each face. The polyelectrolyte is neutralized with counter-ions, balancing the charge of the anions covalently fixed to the membrane. When an IPMC is hydrated and stimulated by a small voltage (1-5 V), both the fixed anions and the mobile counter-ions are subjected to the electric field. The counter-ions diffuse toward one of the electrodes and, as a result, the composite undergoes a fast bending deformation toward the anode. The bending is the result of increased stiffness along the cathode and decreased stiffness along the anode. Examples of IPMCs include perfluorosulfonate (Nafion) and perfluorocarboxylate (Flemion) coated with metal ions such as platinum or gold. Another subcategory of ionic EAPs comprises ionic polymer gels (IPG) such as polyacrylonitrile (PAN).
Electrical stimulation of the tissue of a patient's body for medical purposes is well known. An example of a device for this purpose is the cardiac pacemaker. In the pacemaker context, as well as other body stimulation contexts, the stimulation is delivered to a desired body site by an electrode-carrying lead.
Interactions between the lead and the patient's body can vitiate the desired effects of the stimulation. For example, material reactions and healing may encourage fibrosis. In the pace making context, fibrosis is believed to be a major factor in the increase in chronic stimulation threshold that is usually experienced. Also, mechanical trauma may result in inflammation of the tissue to be stimulated. Such inflammation may alter the response of the tissue to the stimulation energy, both acutely and chronically.
Other interactions between the lead and the body, while not directly affecting the response of the tissue to the stimulation energy, can result in the occurrence of undesirable events. For example, the placement of a pacing lead may induce a cardiac arrhythmia. Furthermore, the presence of the lead may also promote thrombus formation. These interactions have been long recognized and efforts have been made to ameliorate their consequences. For example, therapeutic agents in the form of drugs may be released in vivo to counter trauma caused by an implanted device such as a cardiac pacemaker lead. Because such trauma typically occurs in the region in which the distal end of the pacing lead contacts the cardiac tissue, a pacing lead may have a cavity or collar at the distal end of the lead containing a drug to counter undesirable interactions between the lead and the tissue. Steroid-eluding leads having a tip electrode housing a variety of matrix materials with a drug being stored in, and dispensed from, the tip electrode, are also well known. Anti-inflammatory steroids may also be embedded within a thin coating of a hydrophilic polymer overlying an implantable porous stimulating electrode. The steroid simply diffuses from the polymeric layer into the adjoining tissue to reduce growth of connective tissue. Body implantable pacemaker leads utilizing an osmotic pump to control dispensing of a therapeutic agent or drug are also known.